This invention relates to the use of delay through a delay line for measuring the dielectric properties of materials surrounding the delay line, in particular for the measurement of the dielectric constant where changes of conductivity affect the delay time.
The concept of measuring dielectric constant through the use of a delay line is taught by Friedman in U.S. Pat. No. 3,965,416. Friedman further teaches the delay line can be integrally coupled as a portion of an oscillator and produce a frequency that is a function of the dielectric constant. The measured frequency can then be used to determine the dielectric constant.
Woodhead, et al., in U.S. Pat. No. 5,148,125, teaches that the delay line can specifically be used as the feedback element in a simple oscillator structure and produce an output frequency that is proportional to the dielectric constant. Woodhead specifically applies this technique to the measurement of soil moisture. While the teachings of Woodhead provide the teaching necessary to construct a simple sensor, the sensor tended taught by Woodhead tends to be heavily influenced by the electrical conductivity of the medium. The conductivity of soil is dependent upon soil salts, temperature and composition. The losses in the medium tend to slow the rise time and therefore make the delay longer than just the propagation through the delay line would indicate. In a sensor derived from Woodhead""s teachings, the sample would appear to be moister that it actually was. Woodhead""s teachings acknowledge this in the following statement, xe2x80x9cthe influence of losses due to solid conductivity may be reduced by insulating the line, although the volume of soil sampled and therefore the influence of soil moisture via capacitance is also reduced.xe2x80x9d The need to compensate for temperature effects on the dielectric constant is well known, but Woodhead""s teachings fail to instruct in how to compensate for these changes.
Hocker U.S. Pat. No. 5,430,384) teaches the use of soil resistivity measurement to determine soil moisture content. However, fertilization adds ions that change the conductivity and hence change the moisture measurement reading.
Feuer, in U.S. Pat. No. 5,445,178, also teaches that xe2x80x9cthe use of an LC oscillator can minimize the adverse effects of the conductivity variances in the medium being monitored, because the resistance of the medium, (and, thus, the medium""s conductivity) has minimal or now effect on the resonant frequency of an LC oscillator circuit.xe2x80x9d While this method is effective in many cases, the frequencies are generally very high. This method is thus limited by the capacitor""s size and therefore, the propagation time across the capacitive plate or rod further limit the effective measuring area. Also, as the length of the capacitor grows, the resistance decreases until it can again affect the accuracy of the instrument.
It is the object of the present invention to provide a dielectric monitor which allows the measurement of moisture content or content of any other materials with a high dielectric constant averaged over an extended volume of material where the sensor has the ability to compensate for some level of variable conductivity.
Accordingly, the invention provides a method where the rise time of the pulse arriving from the transmission line is used to compute a correction for the delay time induced by the pulse degradation caused by loss of signal strength due to the conductivity of the sample. It also provides a method to compute a correction factor for temperature effects. We also demonstrate three independent sensing apparatus that each utilizes the correction method above.